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Photogrammetry: DTM Extraction & Editing

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Page 1: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Photogrammetry: DTM Extraction & Editing

Page 2: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

How can one determine the x, y, and z of a location?

Approaches to DTM Extraction

• Ground surveying • Digitized topographic maps • Traditional photogrammetry

–Hardcopy vs. softcopy approach • Radar (e.g., SRTM 2003 and eSRTM 2014) • LIDAR

Page 3: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Photogrammetry

• The science of making reliable measurements by the use of photographs and especially aerial photographs.

• Challenges and solutions: – Geometric distortions (transformation) – Relief displacement (ortho-rectification) – Obscured targets (true-orthorectification)

Page 4: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Distortion

• Distortion: shift in the position of an image on a photograph that alters the perspective characteristics of the image.

• Displacement: shift in the position of an image on a photograph that does not alter the perspective characteristics of the photo

Page 5: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Types of Distortion

• Film and print Shrinkage • Atmospheric Reaction of light rays (refraction) • Image motion • Lens Distortion The effects of film shrinkage, atmospheric

refraction are usually negligible in most cases.

Page 6: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Geometric Distortions

• External errors –Altitude changes –Attitude changes (roll, pitch, and yaw)

• Internal errors

–e.g., lens distortion, earth rotation

Page 7: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?
Page 8: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Methods of Correcting Geometric Distortion

• Affine Transformation (aka linear or first-order transformation)

• Higher order polynomial transformation

Page 9: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Orthophoto & Ortho-rectification Orthophotos - orthographic photographs

• Photographs that do not have distortions nor displacements.

True orthophotos: http://www.sharpgis.net/page/True-Orthophoto-Generation.aspx

Page 10: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

How to tell if triangles are similar • AAA are congruent (i.e.,

coincident) • SSS in same proportion • SAS (proportional sides next to

congruent angle)

Page 11: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Known: x0, y0, x1, y1, and x Find: y

Page 12: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Basic Aerial Photography Geometry •Fiducial marks •Principal point •9” x 9” (or 228.6mm x 228.6mm)

Page 13: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Geometric Components of Relief Displacement

Hf

S=

1

Hrhd =

rdHh =

d = relief displacement h = object height r = radial distance between location and PP on photo H = flying height

1/S: photo scale f: focal length of camera H: flying height

(Exposure Station)

Page 14: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Calculating Relief Displacement

d = relief displacement h = object height r = radial distance between

location and PP on photo H = flying height

H

h

r d

R D

Page 15: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Relief Displacement

RD changes the measured distances and angles on photos.

Page 16: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Correcting for Relief Displacement: Orthorectification

Hrhd =

d = relief displacement h = object height r = radial distance between location and PP on photo H = flying height

Page 17: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Exercise

Hrhd =

rdHh =

d = relief displacement h = object height r = radial distance between location and PP on photo H = flying height

• Given a photo with known flight height (e.g., 1220 m)

• Demonstrate the steps to measure the height of a building on the photo.

Page 18: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Image Parallax • the apparent displacement or the

difference in apparent direction of an object as seen from two different points not on a straight line with the object.

pa = xa – x’a pa = parallax of point A xa = x coor of a on left photo x’a = x coor of a’ on right photo

Page 19: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?
Page 20: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

x x’

pa = xa – x’a

Page 21: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Image Parallax

f

B

h

da d’a

f

B

h

da d’a

pa = xa – x’a

pa = da + d’a

pa = da – d’a

Ground Target

1 2

1 2 Perspective center 1 Perspective center 2

2 1

Page 22: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Calculating Object Height & Location from Parallax

aA p

fBHh ×−=

a

aA p

xBX =

Parallax Equations:

a

aA p

yBY =

pa = parallax of A xa = x coor of A on left photo XA = ground coor of A hA = height of A B = air base H = flying height

Page 23: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

H

f

B

h

xa xa’

Xa

H

h

ya

Ya

f

Values of Xa, Ya, and h?

BhH

xxf

aa

−=

+ ' fx

hHX aa =− f

yhH

Y aa =−

Page 24: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Measuring Parallax

Based on a stereopair of photos • Floating half marks • Parallax wedge

Page 25: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Digital Photogrammetry: Softcopy Photogrammetric Systems

• Scanned stereopair photos • Interior and exterior orientations

–Camera & photo parameters – Flight parameters –GCPs

• Image matching –Tie points –Algorithms

• Generate DEM and orthophotos

Page 26: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Collinearity Condition & Equations

• Alignment of exposure station (O), object location on the photo (p), and object location on the ground (P).

• If collinearity condition is achieved on both photos in a stereopair then the ground X, Y, Z can be computed from x and y within the image coordinate system on both photos.

• Six exterior orientation parameters

• Collinearity equations can be derived using GCPs.

• Inertial Measurement Unit (IMU)

Page 27: Photogrammetry: DTM Extraction & Editingweb.pdx.edu/~jduh/courses/geog493f14/Week02.pdf · Photogrammetry: DTM Extraction & Editing . How can one determine the x, y, and z of a location?

Photogrammetry / Structure From Motion Software

• VisualSFM (open source) (web) • Agisoft PhotoScan (web)

• Standard educational edition $59 • Professional educational edition $549

• PhotoModeler (web) • PhotoModeler $1145 • PhotoModeler Scanner $2495 • PhotoModeler Motion $3495